The present invention relates to an Nixe2x80x94Fe sputtering target for forming magnetic thin films, and specifically to an Nixe2x80x94Fe sputtering target for forming ferromagnetic thin films.
In recent years, magnetic recording devices for computers, such as hard disks, have rapidly been downsized, and their capacities have been increased. The recording density of such devices is estimated to reach 20 Gb/in2 in a few years. Therefore, conventional induction-type heads used as playing heads have approached their limit, and alternatively, magneto-resistance-effect-type (MR) heads have begun to be used. Use of the MR heads is expected to grow rapidly in the future in a worldwide scale accompanying the growth of the personal computer market. In coming years, the practical use of giant magneto-resistance-effect-type (GMR) heads, expected for their further higher density, will be realized.
Nixe2x80x94Fe alloys have been studied for use as a ferromagnetic film of the spin-valve film used in GMR heads.
Nixe2x80x94Fe alloys are normally produced by sintering or melting. However, conventional Nixe2x80x94Fe alloys release a large amount of gases, produce a large number of particles during sputtering, and have the problem of corrosion resistance. Also, their magnetic properties are not found to be satisfactory.
It is an object of the present invention to provide means for forming a ferromagnetic film which releases less gases, produces fewer particles during sputtering, and has good magnetic properties.
In order to solve the above problems, the inventors of the present invention repeated studies, and discovered that impurity elements, in particular, oxygen, sulfur, carbon, nitrogen, and hydrogen increased the release of gases and the production of particles, and that such impurities were the cause of lowered corrosion resistance. In addition to the above, the inventors discovered that the magnetic properties depended mainly on the crystalline structure of the thin film, and that the magnetic properties were improved when the crystals were large columnar crystals.
According to the present invention, and based on the above stated findings, an Nixe2x80x94Fe alloy sputtering target for forming magnetic thin films is provided such that it has an oxygen content of 50 ppm or less, a sulfur content of 10 ppm or less, a carbon content of 50 ppm or less, and a content of total metal impurities other than the alloy components of 50 ppm or less. Preferably, the content of oxygen is 10 ppm or less, the content of sulfur is 1 ppm or less, the content of carbon is 10 ppm or less, and the content of total metal impurities other than the alloy components is 10 ppm or less.
In addition, the Nixe2x80x94Fe alloy sputtering target has a nitrogen content of 10 ppm or less and a hydrogen content of 1 ppm or less. Preferably, the content of nitrogen is 1 ppm or less, and the content of hydrogen is 0.5 ppm or less.
According to another aspect of the present invention, a magnetic thin film formed by sputtering an Nixe2x80x94Fe alloy target as described above is also disclosed.
According to yet another aspect of the present invention, a method of manufacturing the above discussed Nixe2x80x94Fe alloy sputtering target is provided. The method includes the step of alloying by: melting high-purity Ni and high-purity Fe obtained by dissolving material Ni and Fe in hydrochloric acid to form an aqueous solution of chlorides; removing impurity metal ions by allowing the aqueous solution of chlorides to contact an ion exchange resin; evaporating to dryness or concentrating the obtained solution; dissolving it in water to form an aqueous solution of chloride having pH between 0 and 3; removing organic matters in the solution using activated charcoal; and conducting electrolytic refining of the aqueous solution as an electrolytic solution. The method also includes the step of casting the obtained alloy.
In addition, the method of manufacturing an Nixe2x80x94Fe alloy sputtering target can include obtaining Ni or Fe by electrolytic refining and subjecting it to degassing.
An Nixe2x80x94Fe alloy sputtering target for forming magnetic thin films according to the present invention comprises an Nixe2x80x94Fe alloy containing 70% by weight or more Ni. Although typical examples are two-component alloys of Ni and Fe, the examples further include alloys also containing Co, Cr, Rh, Nb, or Ta.
In the Nixe2x80x94Fe alloy sputtering target according to the present invention, the contents of impurities, i.e., elements other than Ni and Fe, are reduced. In particular, the contents of oxygen, sulfur, carbon, nitrogen, and hydrogen are reduced as much as possible, because such elements lower the corrosion resistance of the target, cause particles to occur, and deteriorate magnetic properties. Among these elements, oxygen and sulfur especially lower the corrosion resistance. Furthermore, since oxygen makes crystals finer and nitrogen deviates crystal orientation, both cause the deterioration of magnetic properties. Carbon also causes particles to occur. Therefore, the content of oxygen should be 50 ppm or below, preferably 10 ppm or below; the content of sulfur should be 10 ppm or below, preferably 1 ppm or below; and the content of carbon should be 50 ppm or below, preferably 10 ppm or below.
Furthermore, the content of nitrogen should be 10 ppm or below, preferably 1 ppm or below; and the content of hydrogen should be 1 ppm or below, preferably 0.5 ppm or below.
Exceeding the above contents is not preferred because of increase in occurrence of particles, significant lowering of corrosion resistance, and marked deterioration of magnetic properties.
The inventor of the present invention found that the impurities in the Nixe2x80x94Fe alloy were originated from electrolytic Ni and Fe materials. The inventor carried out the high purification of each of the Ni and Fe materials.
By the combination of ion exchange and electrolytic refining, activated charcoal treatment, and degassing as required in the method for high purification of Ni and Fe materials, extremely high-purity Ni and Fe can be obtained.
For example, the following method can be used.
High-purity Ni and high-purity Fe can be obtained by: dissolving material Ni and Fe in hydrochloric acid to form an aqueous solution of chlorides; removing impurity metal ions by allowing the aqueous solution of chlorides to contact an ion exchange resin; evaporating to dryness or concentrating the obtained solution; dissolving the concentrated solution in water to form an aqueous solution of chloride having pH between 0 and 3; removing organic matters in the solution using activated charcoal; and conducting electrolytic refining of the aqueous solution as an electrolytic solution.
Although the purity of the Ni and Fe utilized are not particularly limited, those of three-nine purity (99.9%) which are normally marketed are sufficient.
The above Ni material, or Fe material, is charged in a vessel and dissolved in hydrochloric acid. The type of hydrochloric acid utilized is not particularly limited, for instance, industrial low purity hydrochloric acid may be used. This is because impurities contained in hydrochloric acid can also be removed by practicing the present invention.
Equipment for dissolving Ni or Fe is preferably provided by a cooling tower for the effective use of hydrochloric acid and a hydrogen chloride gas recovering unit. The material of the equipment is preferably quartz, graphite, Teflon, or polyethylene.
The dissolving temperature is 10 to 100xc2x0 C. If the temperature is less than 10xc2x0 C., the dissolving rate decreases, and if the temperature is more than 100xc2x0 C., evaporation becomes vigorous and loss of the aqueous solution increases.
When Ni is highly purified, the Ni solution is extracted, concentrated, and adjusted to have a hydrochloric acid concentration of 5 to 12N by adding hydrochloric acid. A hydrochloric concentration of less than 5N, or more than 12N, is not preferred because Co is not absorbed and removed by the ion exchange resin.
The above nickel chloride solution adjusted to have a hydrochloric acid concentration of 5 to 12N is allowed to contact an anion exchange resin to absorb impurities in the solution. The ion exchange resin used in the present invention is not particularly limited if it is an anion exchange resin. Examples include DOWEX 1xc3x978, DOWEX 2xc3x978 (Muromachi Chemicals Co., Ltd.), and DIAION SA 10A.
Since Co, Fe, and U form chloride complexes in concentrated hydrochloric acid, and are present as anions, they are adsorbed on anion exchanged resins. On the other hand, since Ni and alkali metals such as Na, K and Th, present as impurities do not form chloride complexes, they are not adsorbed, but flow out of the column.
At this time, for proper separation of Ni and Co, the flow rate of the solution is preferably SV=0.01 to 1. Here, xe2x80x9cSVxe2x80x9d stands for space velocity, and is the quantity of the solution per hour divided by the volume of the packed ion exchange resin. If SV is 0.01 or less, the productivity decreases, and if SV is 1 or more, Fe and Co are not sufficiently adsorbed and high-purity Ni cannot be obtained.
Through the above operations, Co and U impurities are separated from Ni.
Co and U adsorbed on the anion exchange resin can be eluted easily by the use of hydrochloric acid of a concentration less than 1N. Therefore, the anion exchange resin can be recovered by eluting Co and U at a suitable time, considering the adsorption capacity of the anion exchange resin.
The purification of Fe can be performed in the same manner as the purification of Ni.
Since the nickel chloride, or iron chloride solution, eluted from the ion exchange resin has a high hydrochloric acid content, it cannot be used for electrolytic refining as it is. Therefore, by evaporating to dryness or concentrating the eluted nickel chloride, or iron chloride, solution and adding pure water, an aqueous solution of a pH between 0 and 3 is obtained and used as the electrolytic solution.
The evaporation to dryness, or concentration, step may be carried out with a rotary evaporator or the like. The temperature for evaporation to dryness or concentration is 80xc2x0 C. or above, preferably 100xc2x0 C. or above. A temperature below 80xc2x0 C. is not preferred because evaporation to dryness or concentration takes a long time. Evaporation to dryness or concentration carried out with an aspirator under a weakly reduced pressure will reduce the time taken. The material of the equipment used for the evaporation to dryness or concentration step is preferably quartz, graphite, or Teflon. Hydrochloric acid gas produced during evaporation to dryness or concentration can be cooled and concentrated for reuse as hydrochloric acid for dissolving Ni or Fe.
Small quantities of organic substances (styrene, divinylbenzene, amines, etc.) may flow out of the ion exchange resin and mix in the solution. An activated charcoal treatment is carried out to remove such organic substances. Since activated charcoal may contain impurities, it is preferable to use activated charcoal after treating with an acid such as hydrochloric acid to remove such impurities. Although the activated charcoal treatment is normally carried out after the nickel chloride, or iron chloride, solution and water is added to adjust the pH to 0 to 3, the activated charcoal treatment is not necessarily carried out in this order, but may be carried out at any time between the ion exchange and electrolytic refining steps.
The pH of the electrolytic solution comprising the aqueous solution of high-purity Ni or high-purity Fe is 0 to 3, preferably 0.5 to 2. A pH below 0 is not preferred because a large quantity of hydrogen will be produced and current efficiency will decrease. A pH above 3 is also not preferred because Ni or Fe precipitates as nickel hydroxide or iron hydroxide.
The concentration of Ni or Fe in the electrolytic solution during electrolytic refining is 10 to 100 g/l, preferably 20 to 80 g/l. A concentration less than 10 g/l is not preferred because a large quantity of hydrogen will be produced, current efficiency will decrease, and the concentration of impurities in electrodeposited Ni or Fe will increase. A concentration more than 110 g/l is also not preferred because nickel chloride or iron chloride will deposit affecting the electrodeposited state.
The range of current densities are preferably between 0.01 and 10 A/dm2. A current density less than 0.01 A/dm2 is not effective, because the productivity will decrease. A current density more than 10 A/dm2 is also not preferred because the concentration of impurities will increase and current efficiency will decrease.
Temperature during electrolysis is in a range between 10 and 90xc2x0 C., preferably between 35 and 55xc2x0 C. Temperature below 10xc2x0 C. is not preferred because current efficiency will decrease, and temperature above 90xc2x0 C. is also not preferred because the evaporation of the electrolytic solution will increase.
As the anode, crude Ni or crude Fe is used.
As the cathode, an Ni, Fe or titanium plate is used.
The material for the electrolytic vessel is preferably polyvinyl chloride, polypropylene, or polyethylene.
In electrolytic refining, it is preferred to isolate the cathode from the anode with a diaphragm or an anion exchange membrane, and to feed to the cathode side at least intermittently the aqueous solution of high-purity Ni or high-purity Fe (acting as the catholyte) refined by ion exchange and activated-charcoal treatment, so that impurities eluted from the anode do not migrate toward the cathode, and also to extract from the anode side at least intermittently the anolyte having a high impurity content. The quantity of the catholyte fed at this time is preferably the same or more than the quantity of the anolyte extracted.
The diaphragm or the anion exchange membrane available in the present invention is not particularly limited. Examples of diaphragms include Filter Cloth P-2020 and PP-100 (Azumi Filter Paper Co., Ltd.) and Tevylon 1010, and examples of anion exchange membrane include Jonac MA-3475 (Muromachi Chemical co., Ltd.).
The extracted anolyte can be recycled and reused by adjusting the hydrochloric acid concentration to 5 to 12N, then allowing it to contact an anion exchange resin, whereby electrolytic refining can be carried out continuously.
For the purpose of the present invention, xe2x80x9cat least intermittentlyxe2x80x9d means xe2x80x9ccontinuously or intermittentlyxe2x80x9d.
Thus a trace of Th, or alkali metals such as Na and K, remaining in the electrolytic solution after the above electrolytic refining can be separated from Ni or Fe.
By the heat treatment of recovered electrodeposited Ni or electrodeposited Fe in a reducing atmosphere, such as H2, gas components such as oxygen can be removed. The temperature for the heat treatment is preferably 800 to 1550xc2x0 C., and more preferably 100 to 1500xc2x0 C. If the temperature is below 800xc2x0 C., degassing takes a long time, while if the temperature is above 1550xc2x0 C., Ni or Fe is partially melted to cause contamination from the crucible.
More preferably, electron-beam melting is performed. In electron-beam melting, an electrode (here, electrodeposited Ni or electrodeposited Fe) is first produced, and it is melted again to obtain high-purity ingots. Volatile components evaporate while an electrode is melted at a high temperature under a high vacuum. For example, when the melting quantity is 5 kg, electron-beam melting is performed under the following conditions: current: 0.7 A; voltage: 20 kV; degree of vacuum: 10xe2x88x925 mmHg; and time: 2 hr.
High-purity Ni and high-purity Fe obtained by the above methods are melted and alloyed, then cast. The obtained Nixe2x80x94Fe alloy ingot is machined to fabricate a target for sputtering. Basically, the purity of the target is the same as the purity of the ingot.
By sputtering the thus obtained target, a magnetic thin film can be formed.
The Nixe2x80x94Fe alloy magnetic thin film obtained by sputtering has the purity same as the purity of the target, that is, an oxygen content of 50 ppm or less, a sulfur content of 10 ppm or less, a carbon content of 50 ppm or less, and a total content of metal impurities other than alloy components of 50 ppm or less. Furthermore, the crystalline structure is columnar.
The magnetic properties of such a magnetic thin film are especially good because the crystalline structure is columnar.
Although the present invention is described below in detail referring to examples, the present invention is not limited to these examples.